Solid state relays perform functions similar to electromagnetic relays, but are more reliable, since there are no moving parts. Since the turn on and turn off times of a solid state relay are controllable, the solid state relay also minimizes the generation of switching transients. Solid State Relays (SSRs) with the output stage built on power MOSFETs are easily controllable and are commonly used for general purpose switching of signals and low power AC/DC loads.
Generally, a Solid State Relay (SSR), also known as a photo-MOSFET, includes a light-emitting diode (LED), a photo-sensitive FET driver and high-voltage MOSFETs. The relay turns on (i.e., contact closes) when LED is on. The relay turns off (i.e., contact opens) when LED is off. SSRs are commonly used for general purpose switching of signals and low power AC/DC loads. SSRs also provide both switching and galvanic isolation functions.
Referring to FIG. 1, a functional diagram of a conventional solid state relay (SSR) 100 is shown. The SSR consists of an LED 104, photovoltaic cells 106, turn off circuit 108 and a pair of power MOSFETS Q1, Q2. The two MOSFETs in the SSR 100 are serially connected in reverse together. Therefore, when the SSR 100 is conductive, both of the MOSFETs conduct bi-directionally. When the LED 104 lights up based on a control signal from driving circuit 112, voltage will build up in the photovoltaic cells 106. This voltage is applied between the gates and sources of the MOSFETs. It charges the gate capacitance and turns on the MOSFETs, and the SSR is on. When LED 104 is off due to the removal of the control signal from the driving circuit 112, the photovoltaic cells 106 stop charging the gate capacitor of the MOSFETs and the internal discharger circuit or turn off circuit 108 is automatically activated, forcing the gates of MOSFETs to discharge. As a result, the gate-source voltage immediately drops and the MOSFETs turn off.
Presently used methods for driving SSRs require a significant amount of energy. This inordinate energy consumption is undesirable and may lead to design difficulties when hundreds of SSRs are used in a design application.
Various embodiments address at least some of these problems.